In a storage device, Input Output accesses sometimes concentrate on a particular narrow storing region. As an exemplary known solution to the above, in a tiered storage system including a Hard Disk Drive (HDD) and a semiconductor memory device, data stored in a region in the HDD on which region IO accesses concentrate is migrated to a Solid State Drive (SSD), which has a high access speed, so that decline in throughput due to the IO accesses concentration can be avoided.
Besides, a storage system predict a time for which such an IO access concentration continues. Specifically, each time an IO access concentration occurs, the duration time of the IO access concentration is measured; the total of the duration times in a unit of a relatively long span such as one day is calculated; and the duration time of the next IO access concentration is predicted by referring to the total.
[Patent Literature 1] Japanese Laid-open Patent Publication No. HEI 5-225167
[Patent Literature 2] Japanese Patent No. 5707540
However, a typical storage device predicts a duration time of an IO access concentration, using a total duration time of a long span, such as the sum in a unit of one day. Accordingly, in cases where the distribution of the duration time of IO concentration changes in the course of a day, analysis is to be carried out using a total having both data before and data after the change. This lowers the accuracy in predicting after the tendency of an IO access concentration changes, so that predicting related to an IO access concentration comes to be inaccurate.